1. Field of the Invention
The present invention relates in general to pistons of an internal combustion engine and method of producing same. More specifically, the present invention is concerned with the pistons of a type having a bowl at a top of the piston crown, and method of producing same.
2. Description of the Prior Art
Usually, pistons of internal combustion engine are made of aluminum alloys because the alloys are light weight and have the required strength. The piston has at a top of the crown thereof a recess or bowl which serves as a combustion chamber, and has at a predetermined portion thereof a piston pin hole through which a piston pin passes for pivotally connecting a connecting rod to the piston. For producing an engine power, the piston is forced to reciprocate in a cylinder formed in a cylinder block.
Under operation of the engine, the top surface of the piston crown receives a marked thermal shock. In fact, it has been revealed that during engine operation a small but certain deformation takes place in the piston crown due to the powerful combustion pressure and inertial force that are inevitably produced when the piston makes the reciprocating motion.
The piston pin functions to transmit the combustion pressure applied to the piston crown to the connecting rod and thus the deformation of the piston crown is greatly dominated by the position of the piston pin. That is, the combustion pressure is applied in an axial direction of the piston and thus a certain bending moment with a center at the piston pin is inevitably produced on the crown top surface. This means that under explosion stroke of the piston, portions of the crown top surface that extend along the axis of the piston pin are applied with a tensile stress and portions of the crown top surface that extend along an axis perpendicular to the piston pin axis are applied with a compressive stress. While, as to the thermal shock inevitably applied to the crown top surface, the portions of the crown top surface that extend along the axis perpendicular to the piston pin axis are applied with a marked thermal stress due to the density distribution of the piston. This means that first portions of a peripheral edge of the bowl (viz., combustion chamber) formed in the crown top surface that are located along the axis of the piston pin are applied with a tensile stress and second portions of the peripheral edge that are located along the axis perpendicular to the piston pin axis are applied with such marked thermal stress. Thus, hitherto, various measures have been proposed and actually employed for providing the first portions with a sufficient mechanical strength and the second portions with an anti-thermal stress structure.
Japanese Patent First Provisional Publication 2-113169 shows a measure for such need. That is, in the measure, the first portions are strengthened or hardened and the second portions are softened.
However, even the measure proposed by the publication has failed to exhibit a satisfied result because of a boundary that is inevitably formed between the hardened and softened portions. As is known, such boundary part tends to be subjected to a stress concentration, that lowers the mechanical strength of the piston.
It is therefore an object of the present invention to provide a piston of internal combustion engine, which is free of the above-mentioned drawbacks.
It is another object of the present invention to provide a method of the piston.
According to a first aspect of the present invention, there is provided a piston of an internal combustion engine. The piston comprises a crown constructed of an aluminum alloy and having a bowl at a top surface thereof, the bowl serving as a combustion chamber; boss portions projected downward from the crown, the boss portions having aligned piston pin holes respectively, the aligned piston pin holes having a common first axis; and a generally annular reinforced zone provided on the top surface of the crown and extending around a peripheral edge of the bowl, wherein the mechanical strength of the reinforced zone gradually lowers as the distance from the first axis of the piston pin holes increases.
According to a second aspect of the present invention, there is provided a method of producing a piston of an internal combustion engine. The piston is constructed of an aluminum alloy and has piston pin holes at downward projected portions thereof and a bowl at a crown top surface thereof. The method comprises the steps of (a) preparing a blank piston that has no bowl at the crown top surface; (b) placing a fusible ring on the is crown top surface, the fusible ring containing reinforcing materials; (c) fusing the fusible ring and its neighbor of a base material of the blank piston thereby to produce an alloy layer on the crown top surface, the fusing being so made that the amount of the reinforcing materials contained in the alloy layer gradually lowers as the distance from a first axis increases, the first axis being a common axis of the piston pin holes; and (d) machining a bowl at the crown top surface in the area of the alloy layer.